Preparation of thin crystalline films of compound materials has proven difficult where one or more of the component elements are too volatile at the crystal growth temperature to maintain stoichiometry in the film. Examples of materials where this problem exists include high temperature oxide superconductors such as Y1Ba2Cu3ox, Tl2Ba2Ca2Cu3Ox, and variants, other multi-element metal oxides, and compound semiconductors such as GaAs, GaAlAs, GaInAs, and HgCdTe.
Many of the deposition processes, which give excellent crystalline films of single elements or compounds of elements with similar volatilities have difficulty with compound materials whose elements differ widely in volatility. Most of these processes work best at vacuum or near-vacuum pressure. These processes include low pressure chemical vapor deposition (LPCVD), sputtering, E-beam evaporation, molecular beam epitaxy, pulsed laser ablation, and pulsed electron beam ablation.
Several of these conventional processes require an anneal step to be performed at much higher pressure of the volatile element in order to restore the stoichiometry. This procedure is less and less effective as the film thickness is increased.
Other problems inherent in this field include film surface temperature determination and control, problems of handling and risk of contamination of substrates and films due to thermal pastes, and vacuum system contamination and personnel exposure to toxic chemicals. Moreover, a continuing need exists for improved sample throughput and automation, and for the elimination of ablation target overheating and ablation particle contamination of deposited films.
The use of excimer lasers in the deposition of YBCO thin films is described in "Physics of In-Situ Laser Deposition of Superconducting Thin Films" by H. S. Kwok et al., SPIE Vol. 1187, p. 161 (1989). This article notes the significance of target interaction conditions and the properties of the laser-generated plume to the formation of high quality films.
Another excimer laser deposition technique is described in "In-Situ Growth of Superconducting YBa.sub.2 Cu.sub.3 O.sub.y Films By Pulsed Laser Deposition" by J. B. Boyce et al., SPIE Vol. 1187, p. 136 (1989). Here, the authors noted that diminished transport properties correlate with the imperfect alignment and epitaxy of the YBCO and substrate. Experiments were conducted using a ten-sided polygon target holder and a laser pulse frequency of 1 to 10 Hz.
Further laser ablation techniques are described in "Superconducting Bi--Sr--Ca--Cu--O Films Prepared by Laser Ablation" by A. Gupta et al., SPIE Vol. 1187, p. 130 (1989), and "Preparation and Characterization of Pulsed Laser Deposited HTSC Films" by L. Schultz et al., SPIE Vol. 1187, p. 204 (1989).